1. Field of Use
This invention relates to printed wiring boards and more particularly to systems and methods for making changes in printed wiring boards for correcting design errors or adding functional enhancements.
2. Prior Art
As is well known, printed wiring board designs are developed through the utilization of computer aided design (CAD) systems. These systems have interactive graphic and digitizing equipment used by a designer during the physical design and layout phases of a multilayer printer circuit board development.
Such CAD systems are used to provide a digitized output on a magnetic media. The output is in turn applied as an input to photoplotting equipment which generates the PWB artwork. The artwork is then used to fabricate the prototype or production version of the printed wiring board.
Following fabrication, the new printed wiring board is tested to validate or verify that the functional and productibility design requirements have been met. During the design verification cycle, any design errors are identified. The resolution of these errors is made by modifying the physical printed wiring boards by the addition of wires, the cutting of etches and the changing, addition or removal of components.
The elimination of the requirements to add wires and cut etches to the printed wiring boards necessitates the release of a new printed wiring board and associated artwork design. This release process is both lengthy and costly. It normally requires the generation of a new artwork by the CAD system (i.e., all of layers of a multilayer board) and the generation of all new manufacturing/fabrication tools including the recreation of new drill files, router tapes, insertion files, ring out files, etc.
The generation of the new PWB "artwork" involves replotting the locations of all circuit components or modules to be mounted on the board and the wire patterns for interconnecting all of the board components on all of the layers (i.e., plated through holes are used for interconnections between layers).
In general, the replotting function is carried out by the CAD system and necessitates the changing of the entire digital data base which, in turn, requires repetition of the different automated steps required for generation of the new digital data base (e.g. routing programs which route the wires and perform design rule checks for verifying the layout meets established reliability requirements).
The above procedure has been found quite time consuming and costly. To save time, one system employs a method in which the changes are only handled with respect to their effects on the circuitry. This method requires more complicated programming and data management. Also, it results in the documentation of the changes which are implemented by the addition of wires, etch cuts and component replacements. This system is described in the article "The Management of Engineering Changes Using the PRIMUS SYSTEM", by F. P. Mallmann, Proceedings of the 17th Design Automation Conference, Copyright 1980 by the Association for Computing Machinery.
Thus, in this system as in the above systems, until the new PCB design is made available, the process of adding wires and cutting etches to the PWBs being manufactured is required to continue. In this regard, it has been found that PWBs when so modified have been the source of equipment failures. To overcome this problem, some manufacturers have employed other types of processes for modifying each PWB separately. These processes involve spraying or screening metal based substances onto the PWB board. It has been found that the when PWBs modified in this manner have poor reliability either because of break down of the metal based substances or because the metal based substance did not provide the level of conductivity required for high density board construction.
Accordingly, it is a primary object of the present invention to provide a system and method of modifying printed wiring boards which reduces both design cycle time and cost of board construction.
It is a further object of the present invention to provide a system and method for modifying PWBs so as to maintain the same high reliability as the original unmodified PWB.